The present invention relates to a method of forming a concavo-convex pattern in a patterned medium type magnetic recording medium, and a method of manufacturing a patterned medium type magnetic recording medium.
The recording density of magnetic recording media such as hard disks has been increased by various measures including change of the materials composing the disks, reduction of particle size in the magnetic recording layer, miniaturization of a magnetic head, and employment of perpendicular magnetic recording system. Nevertheless, the recording density enhancement is approaching the limit of current technologies.
In order to further increase the recording density, magnetic recording media of a patterned medium type has been proposed, which can be a discrete track type or a bit pattern type, both of which form a fine concavo-convex pattern in the magnetic recording layer.
In one of the methods for manufacturing a patterned medium type magnetic recording medium, a resin material of UV curing (ultra violet light curing), thermoplastic, or thermosetting type is applied on the protective layer and a concavo-convex pattern is formed on the resin material by means of nano-imprint molding. A method of forming the pattern is also implemented by using a photo-resist of a UV curing type resin material and patterning through laser drawing or photo-mask exposure.
FIGS. 2(a) through 2(e) show an outline process of forming a concavo-convex pattern in a patterned medium type magnetic recording medium. FIG. 2(a) shows a stage in which a resin mask layer 16 made of the above-mentioned resin is formed on a protective layer 15 in a magnetic recording medium comprising a soft magnetic layer 11, a seed layer 12, an intermediate layer 13, a magnetic layer 14, and a protective layer 15 formed in this order on a substrate 10. FIG. 2(b) shows a stage in which a concavo-convex structure is formed on the resin mask layer 16 by nano-imprinting molding. FIG. 2(c) shows a stage in which a resist pattern of the resin material is formed by removing the thinner places of the resin mask layer 16 by means of a reactive etching method. Then, the magnetic layer and the protective layer at the exposed places of the magnetic recording medium having a resist pattern of the resin material are removed by means of a reactive etching method, for example. FIG. 2(d) shows a stage in which the magnetic layer and the protective layer at the exposed places of the magnetic recording medium have been removed. Finally, the resin mask layer 16 is removed by exposing to plasma, for example, to complete formation of a concavo-convex pattern in a patterned medium type magnetic recording medium. FIG. 2(e) shows a stage in which formation of the concavo-convex pattern is completed.
The magnetic layer is often composed of a hard to etch material such as cobalt or chromium and etched in a slower etching rate than the resin mask. As a result, etching work may be difficult in forming a resist pattern by a resin mask only. In that case, a hard mask layer exhibiting a lower etching rate than the magnetic layer is formed on the protective layer using titanium, tantalum, or oxide or nitride of those elements, and the concavo-convex pattern is formed in the hard mask layer using the resin mask by means of an ion beam etching method or a reactive ion etching method. Provided the etching rate on the hard mask is sufficiently low in the process of working on the magnetic layer, the hard mask layer can be thin and worked using the resin mask.
FIGS. 3(a) through 3(g) show schematically a process of forming a concavo-convex pattern using a hard mask layer. FIG. 3(a) shows a stage in which a hard mask layer 19 and a resin mask layer 16 formed thereon are formed on a protective layer 15 of a magnetic recording medium similar to the one shown in FIG. 2(a). FIG. 3(b) shows a stage in which a concavo-convex structure is formed in the resin mask 16. FIG. 3(c) shows a stage in which a resist pattern of the resin material is formed by removing the thinner places of the resin mask layer 16 by means of a reactive etching method, for example. FIG. 3(d) shows a stage in which a concavo-convex pattern is formed in the hard mask layer by means of ion beam etching or reactive ion etching. After working on the hard mask layer 19, the resin mask layer 16 is removed by reactive etching technique such as oxygen plasma, ozone plasma or the like. In the case the protective layer 15 is composed of a carbon material such as DLC (diamond like carbon), the protective layer at the places of etched hard mask layer (exposed protective layer places) is processed simultaneously with the resin mask layer through the reactive etching with oxygen plasma or ozone plasma for removing the resin mask layer. FIG. 3(e) shows a stage in which the resin mask layer 16 and the exposed places of the protective layer 15 have been removed. After removing the resin mask layer, the magnetic layer is processed to a concavo-convex structure using a mask of the hard mask layer by means of an ion beam etching method or a reactive ion etching method. Finally, the hard mask layer is removed using a reactive gas exhibiting a low etching rate on the magnetic layer. Thus, a concavo-convex pattern is formed on a patterned medium type magnetic recording medium. FIG. 3(f) shows a stage in which the magnetic layer has been processed into a concavo-convex structure, and FIG. 3(g) shows a stage in which the hard mask layer is removed.
Japanese Unexamined Patent Application Publication No. 2005-056547 discloses a starting article for processing a magnetic recording medium, the article comprising a first mask layer composed of TiN (corresponding to the hard mask layer in the above description) and a second mask layer composed of nickel. Nickel is apt to be etched more easily than cobalt or chromium, which composes the magnetic layer, and functions similarly to the resin mask described above although nickel is an inorganic material and the resin material is an organic material. A resist layer is formed on the second mask layer and after patterning the resist layer, the second mask layer is patterned by ion beam etching, and the first mask layer is patterned by reactive etching using a reactive gas of oxygen. After that, the magnetic layer is patterned by ion beam etching.
In methods of manufacturing a patterned medium type magnetic recording medium based on the conventional technologies, argon gas is commonly used in the ion beam etching for processing the magnetic layer. This ion beam etching is executed by physical etching through collision of argon ions or atoms on the material to be etched. Consequently, degenerated layer due to chemical reaction does not remain and performance degradation in the magnetic layer at the unetched portion scarcely occurs. As for the depth of the processing, however, it is commonly thought that the aspect ratio (depth/width) is about one and striving for minute pattern has its limitation. When the mask layer for the processing is thick, a workable depth further decreases causing difficulty in forming a concavo-convex structure in a fine pattern.
In the case a reactive ion etching method is used for processing the magnetic layer, known etching gases include chlorine-containing gas and a mixture gas of CO and NH3 for etching the metallic material of cobalt, chromium or the like composing the magnetic layer, which are hard to etch. In Japanese Unexamined Patent Application Publication No. 2005-056547, the mixture gas of CO and NH3 is also used for an etching gas.
The chlorine-containing gas used in the etching process forms a chloride with the metals of cobalt and chromium. The chloride vaporizes or is physically removed through collision of ions as progress is made through the working process. In the process, the ions hardly collide with the side walls of the parts being worked and the etching progresses selectively in the perpendicular direction. As a result, the chlorine or chlorides remains on the side walls causing a problem of after-corrosion.
The mixture gas of CO and NH3 forms a carbonyl compound with cobalt and chromium. This compound exhibits a low evaporation temperature and low vapor pressure. As a result, the etching can progress without physical assist such as collision of ions. However, the CO is a gas of strong toxicity and is difficult to handle, and thus is unsuited for industrial production.